This invention relates to a transmission power control system for a mobile communication network.
As a mobile communication network such as a cellular system, use is made of various types of multiple access networks. One of these networks is a code division multiple access (hereinafter simply called CDMA) communication network in which each channel is assigned with a specific code. A modulation wave of a common carrier frequency is spectrum-spread by the use of this code to be transmitted to a common repeater. At a receiving section, synchronization is established for each code to identify a desired channel.
The mobile communication network of such a CDMA network generally comprises a plurality of terminals (mobile terminals or mobile stations) and a base station radio equipment (radio base station) as a repeater. In the mobile communication network, reception energies of uplink communication channels from those terminals communicating with the base station radio equipment are equalized irrespective of locations of the terminals to make the terminals carry out communication by the use of the common carrier frequency. In order to realize the above, transmission power control for the uplink communication channels is carried out as described in the publications of TIA (Telecommunications Industry Association), EIA (Electronic Industries Association), and IS-95.
FIG. 1 is a block diagram showing an entire structure of a mobile communication network of a CDMA network. The mobile communication network comprises a host station 11, first through n-th (n being an integer not smaller than two) base station radio equipments 12.sub.1, 12.sub.2, . . . , and 12.sub.n connected to the host station 11, and a plurality of terminals or mobile terminals 13 (only one being illustrated in the figure). The first base station radio equipment 12.sub.1 comprises channel control sections 30.sub.1, 30.sub.2, . . . , and 30.sub.m, m (m being an integer not smaller than two) in number, similar in structure to one another, an equipment-side transmitter 40, an equipment-side receiver 41, and an equipment-side antenna 42. Each of the base station radio equipments 12.sub.2 through 12.sub.n is similar in structure to the base station radio equipment 12.sub.1. The terminal 13 carries out communication, moving through service areas (not shown) assigned to the first through the n-th base station radio equipments 12.sub.1 to 12.sub.n.
In the above-mentioned mobile communication network, an uplink signal on an uplink communication channel to be transmitted from the terminal 13 to the host station 11 is received, for example, through the equipment-side antenna 42 of the first base station radio equipment 12.sub.1 and demodulated by the equipment-side receiver 41 of the radio equipment 12.sub.1 into a demodulated signal. The demodulated signal is decoded, for example, by the channel control section 30.sub.1 in the radio equipment 12.sub.1 and then transmitted to the host station 11. On the other hand, a signal to be transmitted from the host station 11 to the terminal 13 is encoded, for example, by the channel control section 30.sub.1 of the first base station radio equipment 12.sub.1, modulated by the equipment-side transmitter 40, and thereafter transmitted to the terminal 13 as a downlink signal on a downlink communication channel via the equipment-side antenna 42.
FIG. 2 is a block diagram for describing a conventional transmission power control system for the uplink communication channel in detail. For convenience of illustration, the channel control section 30.sub.1 alone is illustrated in the figure among the channel control sections 30.sub.1 through 30.sub.m in the first base station radio equipment 12.sub.1. The channel control section 30.sub.1 comprises a master control section 31, an equipment-side encoding section 32, an equipment-side decoding section 33, a transmission power control section 34, an equipment-side downlink spread code generating section 35, and an equipment-side uplink spread code generating section 36.
In the base station radio equipment 12.sub.1, the uplink signal on the uplink communication channel from the terminal 13 is received through the equipment-side antenna 42 and demodulated by the equipment-side receiver 41 into the demodulated signal. The demodulated signal is correlation-demodulated by the use of a terminal-specific spread code and then supplied to the transmission power control section 34 and the equipment-side decoding section 33. The terminal-specific spread code is set in the uplink spread code generating section 36 of the channel control section 30.sub.1 selected to communicate with the terminal 13, simultaneously when the host station 11 assigns the communication channel to the terminal 13. The transmission power control section 34 measures a reception Eb/No ratio representative of a density ratio of signal power (reception energy) of the uplink communication channel from the terminal 13 per one bit to noise power (interference energy from other terminals) and compares the reception Eb/No ratio thus measured and a reference Eb/No ratio set in the master control section 31. As the reference Eb/No ratio, a same value is set in all the channel control sections 30.sub.1 through 30.sub.m so as to maintain similar communication quality in all terminals currently communicating.
If the reception Eb/No ratio is greater than the reference Eb/No ratio, this means that the terminal 13 carries out transmission with excessive power. In this event, the transmission power control section 34 produces a transmission power control signal requesting a decrease of uplink transmission power. On the contrary, if the reception Eb/No ratio is smaller than the reference Eb/No ratio, this means that the terminal 13 carries out transmission with insufficient power. In this event, the transmission power control section 34 produces a transmission power control signal requesting an increase of the uplink transmission power. The transmission power control signal produced by the transmission power control section 34 is delivered to the equipment-side encoding section 32.
The equipment-side encoding section 32 encodes the signal transmitted from the host station 11 and superposes the transmission power control signal on the signal. An output signal produced by the equipment-side encoding section 32 is spread by the use of the spread code specific to the base station radio equipment 12.sub.1 to be transmitted through the equipment-side transmitter 40 and the equipment-side antenna 42 to the terminal 13 by the use of the downlink communication channel. The spread code specific to the base station radio equipment 12.sub.1 is produced by each equipment-side downlink spread code generating section 35 in the manner such that the base station radio equipments have different codes to enable the terminal 13 to identify the base station radio equipments. FIG. 3 is a block diagram showing the structure of the terminal 13 illustrated in FIG. 2. The terminal 13 comprises a terminal-side antenna 71, a terminal-side receiver 72, a terminal-side transmitter 73, an output gain control section 74, a terminal-side downlink spread code generating section 75, a terminal-side decoding section 76, a terminal-side encoding section 77, a terminal-side uplink spread code generating section 78, and a speech encoding section 79.
The downlink signal on the downlink communication channel transmitted from the base station radio equipment 12.sub.1 is received via the terminal-side antenna 71 of the terminal 13 and demodulated by the terminal-side receiver 72 into a demodulated signal. The demodulated signal is correlation-demodulated into a correlation-demodulated signal by the use of the spread code specific to the base station radio equipment 12.sub.1 that is set and produced by the terminal-side downlink spread code generating section 75. The spread code set in the downlink spread code generating section 75 is informed from the host station 11 every time when the communication channel is assigned to the terminal 13.
The correlation-demodulated signal is separated by the terminal-side decoding section 76 into a speech signal and the transmission power control signal. The speech signal and the transmission power control signal are delivered to the terminal-side speech encoding section 79 and the output gain control section 74, respectively. The speech signal delivered to the terminal-side speech encoding section 79 is converted by the terminal-side speech encoding section 79 into a speech. On the other hand, an output signal produced by the terminal-side speech encoding section 79 is encoded by the terminal-side encoding section 77, spread by the use of the terminal-specific spread code produced by the terminal-side uplink spread code generating section 78, controlled in output power by the output gain control section 74 in correspondence to the transmission power control signal, modulated by the terminal-side transmitter 73, and transmitted to the base station radio equipment 12.sub.1 via the terminal-side antenna 71.
As described above, the base station radio equipment 121 produces the transmission power control signal for the terminal 13 with reference to the magnitude of the reception Eb/No ratio of the uplink communication channel from the terminal 13, and transmits the control signal to the terminal 13 by the use of the downlink communication channel. The terminal 13 controls the transmission power of the uplink communication channel in response to the transmission power control signal which has been received. Therefore, the reception Eb/No ratio received by the base station radio equipment 12.sub.1 is kept at the level of the reference Eb/No ratio. Thus, the signal quality of the communication channel is maintained.
It is assumed that an abnormal terminal, which does not decrease the transmission power although the transmission power control signal requests the decrease of the transmission power, is present within the service area of the mobile communication network. In this event, when the uplink transmission power of the abnormal terminal becomes excessive, the uplink reception energy of the base station radio equipment 12.sub.1 in the service area is increased. As a consequence, the communication quality of the abnormal terminal is improved. However, the reception energy thus increased acts as a large interference energy on other normal terminals simultaneously communicating. Accordingly, within the service area in which the abnormal terminal is present, the communication quality of each of the other normal terminals present in the same area is deteriorated. In this event, the other terminals try to recover from the deterioration of the communication quality by increasing the uplink transmission power of the individual terminals. This results in an increase in total transmission power and a reduction in subscriber capacity within the service area. Furthermore, in the mobile communication network of a CDMA system, communication in an adjacent service area is also carried out by the use of the common carrier frequency. Therefore, the subscriber capacity may also be reduced in another service area adjacent to the service area in consideration.